As has been described in the preceding section, 5′-GMP2Na and 5′-IMP2Na are important in the fields of seasonings, medicaments, and the like. However, when it is necessary to use both the compounds in combination, it is extremely difficult to prepare a mixture having a predetermined mixed ratio by the mere mixing of crystals of both the respective compounds owing to the differences in crystal natures and powder properties of the compounds, and also the handling of such a mixture is accompanied by various difficulties.
By the way, as methods for producing 5′-GMP2Na and 5′-IMP2Na in the form of their mixed crystals, the following three methods can be mentioned when roughly classified. That is, (1) a method of dissolving 5′-GMP2Na and 5′-IMP2Na in water, followed by precipitating 5′-GMP2Na and 5′-IMP2Na in the form of mixed crystals thereof (hereinafter, abbreviated as I+G mixed crystals) from the resulting solution by cooling, concentrating, and addition of an alcohol (Japanese Patent Publication Nos. 16582/1979 and 4787/1980), (2) a method of dissolving 5′-GMP2Na and 5′-IMP2Na in an aqueous solution containing a hydrophilic organic solvent such as methanol or the like, followed by obtaining mixed crystals of 5′-GMP2Na and 5′-IMP2Na (i.e., I+G mixed crystals) from the resulting solution, and a crystallization method wherein an organic solvent is added to an aqueous mixed solution of 5′-GMP2Na and 5′-IMP2Na (Japanese Patent Publication No. 12914/1965), and (3) a method wherein an aqueous solution containing 5′-IMP2Na is gradually added to a slurry solution in which 5′-GMP2Na is present as the bottom body (Ger., Bodenkörper), whereby I+G mixed crystals are formed (Japanese Patent Publication No. 215494/1991 and Japanese Patent No. 2770470).
On the other hand, 5′-GMP2Na and 5′-IMP2Na are known to form I+G mixed crystals in an aqueous solution containing a hydrophilic organic solvent such as methanol or in a mere aqueous solution, while 5′-GMP2Na is incorporated into the crystal lattice of 5′-IMP2Na. The X-ray diffraction chart of the mixed crystals shows almost the same pattern as that of 5′-IMP2Na, and it is considered that 5′-GMP2Na having a similar chemical structure to that of 5′-IMP2Na enters the crystal lattice of 5′-IMP2Na, wherein a stable state is maintained by hydrogen bonding. The crystals of 5′-IMP2Na have a good crystal shape, and I+G mixed crystals having the same crystal lattice has almost the same good crystal shape.
At the time when I+G mixed crystals are to be obtained, when according to the above method (1), in order to obtain a product (mixed crystals) having a desirable ratio (weight ratio) of 5′-IMP2Na and 5′-GMP2Na (hereinafter, the ratio (weight ratio) of both the compounds is abbreviated as I/G ratio), it is necessary to control strictly concentrating drain, feed liquid, and setting conditions of temperature, pressure, and the like, in the case of the crystallization by concentrating, and more strict control of the composition of crystallizing liquid is required in the case of the crystallization by cooling, since the composition of crystallizing liquid changes continuously. Thus, there is the problem that apparatuses and process controls concerned get complicated in both cases. According to the method (2), crystallization can be effected in high recovery yields, but when carried out industrially, an expensive explosion-preventing equipment is required because an organic solvent is used, so that there is the disadvantage that the production cost increases. Moreover, it is difficult to control crystallizing conditions, and there is the problem that GMP is generated depending on the crystallizing conditions, and, in turn, the separability of the resulting crystals decreases. According to the method (3), it is necessary to keep separate the raw materials 5′-IMP2Na and 5′-GMP2Na prior to crystallization, and hence the number of equipments increases for avoiding the mixing before crystallization.